Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) have been used for various applications including, but not limited to use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example.
An HEMT is a field effect transistor with a junction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AlGaN barrier layer and the GaN buffer layer. The 2DEG forms the channel instead of a doped region such as in a MOSFET (metal oxide semiconductor field effect transistor). Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. Without further measures, such a construction leads to a self-conducting, i.e., normally-on, transistor. That is, the HEMT conducts in the absence of a positive gate voltage.
One desirable attribute of transistors such as HEMTs is high breakdown voltage. An increased breakdown voltage allows a transistor to handle larger voltages associated with power switching applications, for example. To this end, conventional normally-on GaN-based HEMTs typically make use of a top field plate connected to the source terminal in order to lower the electric field peaks within the device, which in turn increases the breakdown voltage of the device. The top metal field plate is disposed above the gate electrode and insulated from the gate electrode by a dielectric material. The top metal field plate increases the breakdown voltage of the transistor by directing electric field lines away from the gate edge. However, the effectiveness of top-side metal field plates is limited because the field plate cannot be placed at very close distances to the most vulnerable areas of the gate electrode. It is desirable to have a more efficient field plate which increases the breakdown strength of a GaN HEMT by shaping the electric field in such a way to lower the maximum electric field peaks and to enhance the breakdown strength of the device.